What Is Childhood Leukemia?

What Is Childhood Leukemia? Cancer starts when cells in the body start to grow out of control. Cells in nearly any part of the body can become cancer. To learn more about cancer and how it starts and spreads, see What Is Cancer?1 For information about the differences between childhood cancers and adult cancers, see Cancer in Children2. Leukemias are cancers that start in cells that would normally develop into different types of blood cells. Most often, leukemia starts in early forms of white blood cells, but


Chronic myeloid leukemia (CML):
Also called chronicmyelogenous leukemia, CML is rare in children. Treatment is similar to that used for adults (see Treatment of Children With Chronic Myeloid Leukemia [CML] 4 ). For more detailed information on CML, see Leukemia--Chronic Myeloid 5 . q Chronic lymphocytic leukemia (CLL): This leukemia is extremely rare in children. For more information on CLL, see Leukemia--Chronic Lymphocytic 6 . q

Juvenile myelomonocytic leukemia (JMML)
This rare type of leukemia is neither chronic nor acute. It starts in myeloid cells, but it usually doesn't grow as fast as AML or as slowly as CML. It occurs most often in young children (average age of 2 years). Symptoms can include pale skin, fever, cough, easy bruising or bleeding, trouble breathing (from too many white blood cells in the lungs), rash, and an enlarged spleen, liver, and lymph nodes. For info on treating JMML, see Treatment of Children With Juvenile Myelomonocytic Leukemia (JMML) 7 .

Normal bone marrow, blood, and lymph tissue
To understand leukemia, it helps to know about the bone marrow, blood, and lymph systems.

Bone marrow
Bone marrow is the soft inner part of certain bones. It is made up of blood-forming cells, fat cells, and supporting tissues. A small number of the blood-forming cells are blood stem cells.
Blood stem cells go through a series of changes to make new blood cells. During this process, the cells become either lymphocytes (a kind of white blood cell) or other blood-forming cells, which are types of myeloid cells. Myeloid cells can develop into red blood cells, white blood cells (other than lymphocytes), or platelets.

Red blood cells
Red blood cells (RBCs)carry oxygen from the lungs to all other tissues in the body, and take carbon dioxide back to the lungs to be removed.

Platelets
Platelets are actually cell fragments made by a type of bone marrow cell called the megakaryocyte. Platelets are important in stopping bleeding by plugging up holes in blood vessels.

White blood cells
White blood cells (WBCs)help the body fight infections. There are different types of white blood cells: Lymphocytes are mature WBCs that develop from lymphoblasts, a type of bloodforming cell in the bone marrow. Lymphocytes are the main cells that make up lymph tissue, a major part of the immune system. Lymph tissue is found in the lymph nodes, thymus (a small organ behind the breast bone), spleen, tonsils and adenoids, and bone marrow. It is also scattered through the digestive system and respiratory system. There are 2 main types of lymphocytes: B cells and T cells. (ALL, the most common type of childhood leukemia, can start in either B cells or T cells.) For more information, see Childhood Leukemia Subtypes 8 . q Granulocytes are mature WBCs that develop from myeloblasts, a type of bloodforming cell in the bone marrow. Granulocytes have granules that show up as spots under the microscope. These granules contain enzymes and other substances that can destroy germs, such as bacteria. The 3 types of granulocytes -neutrophils, basophils, and eosinophils -are distinguished under the microscope by the size and color of their granules. q Monocytes develop from blood-forming monoblasts in the bone marrow and are related to granulocytes. After circulating in the bloodstream for about a day, monocytes enter body tissues to become macrophages, which can destroy some germs by surrounding and digesting them. q

Start and spread of leukemia
Leukemia starts in the bone marrow. The leukemia cells can build up there, crowding out normal cells. Most often, the leukemia cells spill into the bloodstream fairly quickly. Some types of leukemia can also spread to other parts of the body such as the lymph nodes, spleen, liver, central nervous system (the brain and spinal cord), testicles, or other organs. Some other childhood cancers, such as neuroblastoma 9 or rhabdomyosarcoma 10 , start in other organs and can spread to bone marrow, but these cancers are not leukemia. ____________________________________________________________________________________ Leukemia is the most common cancer in children and teens, accounting for almost 1 out of 3 cancers. Overall, however, childhood leukemia is a rare disease.
About 3 out of 4 leukemias among children and teens are acute lymphocytic leukemia (ALL). Most of the remaining cases are acute myeloid leukemia (AML).
ALL is most common in early childhood, peaking between 2 and 5 years of age. q AML tends to be more spread out across the childhood years, but it's slightly more common during the first 2 years of life and during the teenage years. q ALL is slightly more common among Hispanic and white children than among African-American and Asian-American children, and it is more common in boys than in girls. q AML occurs about equally among boys and girls of all races. q Chronic leukemias are rare in children. Most of these are chronic myeloid leukemia (CML), which tends to occur more in teens than in younger children.
Juvenile myelomonocytic leukemia (JMML) is also rare. It usually occurs in young children, with an average age of about 2. It is slightly more common in boys than in girls. Researchers are now studying the causes, diagnosis, and treatment of childhood leukemia at many medical centers, university hospitals, and other institutions.

Genetics
Scientists are making progress in understanding the changes in the DNA inside bone marrow stem cells 1 that can cause them to develop into leukemia cells. Understanding these gene and chromosome changes can help explain why these cells may grow out of control, and why they don't develop into normal, mature blood cells. Doctors are now looking to use these changes to help them determine a child's outlook and to determine what treatment is likely to be best.
This progress has already led to vastly improved and very sensitive tests for detecting leukemia cells in blood or bone marrow samples. The polymerase chain reaction (PCR) test, for example, can identify very small numbers of leukemia cells based on their chromosome changes. This test is useful in determining how completely the leukemia has been destroyed by treatment, and whether a relapse is likely if further treatment isn't given. Newer tests known as next generation sequencing (NGS) tests, which are just now coming into use, might be able to help with this even more.

Causes, and possibly prevention
Researchers continue to look for possible causes of leukemia in children, which might include a combination of both genetics and environmental exposures.
For example, one theory that has gained some ground in recent years is that some childhood leukemias might be caused by a combination of certain gene changes that happen very early in life (even before birth), along with being exposed to certain germs (particularly viruses) later than normal. This "delayed infection" (after the first year or so of life) might affect the immune system in a way that leads to a second gene change, which in turn might lead to leukemia.
This might help explain why some studies have found that the risk of childhood leukemia seems to be lower in children who were in daycare during their first year of life (which would have exposed them to common infections earlier).
More research is needed to confirm this theory. But if it is confirmed, it might be possible to lower childhood leukemia risk by ensuring children are exposed to certain germs very early in life.

Clinical trials
Most children with leukemia are treated at major medical centers, where treatment is often given in clinical trials 2 to help ensure children get the most up-to-date care. Several important questions are now being studied in clinical trials. Among them are: Why do some children with acute lymphocytic leukemia (ALL) relapse after treatment, and how can this be prevented?
q Are there other prognostic factors 3 that will help identify which children need more or less intensive treatment? What are the best treatment approaches for children with less common forms of leukemia, such as juvenile myelomonocytic leukemia (JMML) 5 and chronic myeloid leukemia (CML) 6 ? q

Immunotherapy to treat childhood leukemia
Immunotherapies are treatments that boost a child's own immune system to help fight leukemia. Some types of immunotherapy have shown a lot of promise in treating childhood leukemia, even when other treatments are no longer working.

Chimeric antigen receptor (CAR) T-cell therapy
In this treatment, immune cells called T cells are removed from the child's blood and genetically altered in the lab to help them attack leukemia cells. The T cells are then given back into the child's blood, where they can seek out the leukemia cells throughout the body.
This technique has shown very encouraging results in clinical trials against some advanced, hard-to-treat cases of ALL. In many children the leukemia could no longer be detected after treatment, although it's not yet clear if these children have been cured.
Doctors are still improving how they make the T cells and are learning the best ways to use them. CAR T-cell therapy is only available at certain major medical centers at this time.

Monoclonal antibody therapy
Antibodies are proteins made by the body's immune system to help fight infections. Man-made versions, called monoclonal antibodies, can be designed to attack a specific target, such as a protein on the surface of leukemia cells.
An example is blinatumomab (Blincyto), a special kind of monoclonal antibody that can attach to 2 different proteins at the same time. This drug brings the leukemia cells and immune cells together, which is thought to cause the immune system to attack the leukemia cells. This drug can be used to treat some types of B-cell ALL.
For more on these treatments, see Immunotherapy for Childhood Leukemia 7 .
Other types of immunotherapy are now being studied as well.

New targeted drugs to treat AML
As researchers have learned more about the gene changes that drive the growth of leukemia cells, they've begun to develop drugs that can target these gene changes. For example, several newer types of targeted drugs are now being used to treat adults with AML, and many of these are now being tested for use in children as well.